Various heat insulation techniques are described, for instance, in the following documents: FR 98/16.791, JP 2 176 299, or WP 97/47174.
Heat insulation can be accomplished by a variety of processes. On shore, or at shallow depths, cellular or wool-type porous cellular materials are used, to stop the convection of low thermal conductivity gas. The compressibility of these porous materials prohibits the technique from being used at relatively great depths.
Another known technique consists in wrapping the line with a first coat of porous material soaked in paraffin, for instance, whose thermal insulation coefficient is lower than those obtained with the gas trapping technique mentioned above, and a second coat of refractory material strengthening the effect of the first coat. However, this kind of solution cannot be used in water.
There are other solutions that are more suitable for use at great depths. For instance, it is possible to use:    solid quasi-incompressible polymer material coatings based on polyurethane, polyethylene, polypropylene etc. which, however, offer relatively average thermal conductivity, insufficient to avoid drawbacks in the event of production stoppages;    coatings of syntactic materials comprising hollow balls containing a gas and resisting the outside pressure, immersed in binders such as concrete, epoxy resin etc., whose conductivity is lower than that of the compact materials, but that are far more costly.
It is also possible to protect the line in which the fluids circulate by an outer line withstanding the hydrostatic pressure. A heat insulation with low thermal conductivity left at atmospheric pressure or placed under vacuum, with partitions placed at regular intervals for safety reasons, is for example interposed in the annulus between them.
It is also well-known to interpose between the line and a deformable protective sheath an absorbing matrix enclosing the line, impregnated with a liquid/solid phase change quasi-incompressible material having a melting temperature higher than that of the surrounding environment and lower than that of the fluids circulating through the line.
The phase change materials (PCM) behave like heat accumulators. They release this energy in the course of solidification (crystallisation) or absorb this energy during fusion, in a reversible manner. These materials can therefore be used to increase the length of production stoppages without any risks of the lines being clogged by premature cooling of their content.
Known examples of phase change materials are chemical compounds of the alkanes family CnH2n+2, such as for instance, n-paraffins (C12 to C60), which represent a good compromise between the thermal and thermodynamic properties (fusion temperature, latent fusion heat, thermal conductivity, calorific capacity) and cost. These compounds are thermally stable in the range of operating temperatures considered and are compatible with use in the marine environment because they are insoluble in water and have a very low toxicity level. Therefore, they are well suited to the thermal insulation of deep water lines.
The change of state temperature of these phase change materials is related to the carbon number of the hydrocarbon chain and can therefore be adapted to a particular application. To obtain a phase change at around 30° C., it is possible, for instance, to use a mixture of paraffins essentially comprising C18 such as Linpar 18-20 marketed by CONDEA Augusta S.p.A.
The use of waxes, normal paraffins, long-chain weakly branched isoparaffins (C30-C40) (1 or 2 branches), of long chain branched alkylcycloalkanes or long chain branched alkyl aromatics, also weakly branched, fatty alcohols or fatty acids, may also be considered.
Above their fusion temperature Tf, phase change materials (PCM) are in the liquid phase and their viscosity is low. To overcome this drawback, which is particularly inconvenient for some applications, particularly in the manufacturing of double wall vessels, or energy storage drums, it is well-known to add a thickening agent, such as silica, to solidify them and prevent leaks from occurring.
Another drawback of phase change materials (PCM) is that their liquid state favours thermal losses by convection.